Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: electrospinning. Includes bibliographical references (p. 95-96).

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In this thesis the production and filtration performance of electrospun nylon-4,6 nanofiber mats was experimentally studied. The average fiber diameter of nylon-4,6 fibers is controlled by altering the polymer concentration in solution. Adding small amount of pyridine to the electrospinning solution controlled fiber beading and other defects. These fibers were then deposited onto a wide variety of conductive and dielectric substrate materials. A corona ion source was used to eliminate surface charging effects observed at the surface of all substrates. The resulting fiber mats, uniform in size and distribution as verified by SEM imaging, are tested for filtration efficiency and pressure drop. A Figure of Merit (FOM) is calculated for each filter produced and compared to high-grade commercial filters.

Specific objective: Guided tissue regeneration (GTR) aims to regenerate the lost attachment apparatus caused by periodontal disease through the use of a barrier membrane. For the GTR procedures to be successful, barrier membranes are required to be present at the surgical site for an extended period of time (weeks to months). Synthetic membranes have the advantage of prolonged presence in a wound site; however, they do not actively contribute to wound healing. Biologic membranes are recognized by the host tissue and participate in wound healing but have the disadvantage of early resorption. Therefore, the goal of this study is to create and characterize a hybrid barrier membrane that contains biologically active fibrin matrix within a synthetic polymeric electrospun scaffold. Method: Fibrin matrices and fibrin-incorporated electrospun scaffold were created from fresh frozen plasma at three different centrifugation conditions 400g for 12 minutes, 1450g for 15 minutes and 3000g for 60 minutes. Following centrifugation, half of the membranes were crosslinked with 1% genipin. Biological stability of these scaffolds was evaluated by resistance to trypsin while their mechanical properties were characterized by MTS Bionix Uniaxial Tensile Testing System. Continuous data was analyzed by ANOVA to detect differences between groups (p=0.05). Results: The addition of an electrospun scaffold to the fibrin matrix led to improvements in the mechanical properties as evidenced by an increase in the modulus (p<0.0001), strain at break (p<0.0001) and energy to break (p<0.0001). The effect of crosslinking was marginal but not statistically significant to the mechanical properties of fibrin matrices or the fibrin incorporated scaffold. However, crosslinking did significantly increase resistance against enzymatic degradation by trypsin (p<0.0001). Lastly, centrifugation speeds at 400g and 1450g showed similar mechanical properties and biologic stability; meanwhile 3000g negatively impacted the properties of the scaffold. Conclusion: Fibrin-incorporated electronspun scaffold exhibits enhanced mechanical and biologic stability compared to fibrin matrices alone. Moreover, crosslinking improves the biologic stability of the novel biomaterial. All these characteristics of the fibrin-incorporated matrix make this membrane a potentially more ideal barrier for GTR procedures to enhance periodontal wound healing.

The objectives of the present work are to produce porous polymeric scaffolds with Poly (ƒÕ-Caprolactone), PCL, by electrospinning. The structure in the electrospun polymer has been characterized by scanning electron microscopy. The effects of process variables such as voltage, solution concentration and deposition distance on the structure have been studied. The physical phenomena associated with the electrospinning process have been highlighted through high speed digital photography. The feasibility of using additives to the solution to control the structure of the porous construct has been examined. The data indicate that a range of structural morphologies can be produced in the electrospun polymer. Solid and hollow sub-micron beads can be produced by electrospraying of dilute solutions. Beyond a critical solution concentration of about 4 wt% PCL, elongational flow stabilizes the fibrous structure and a web of interconnected sub-micron fibers may be obtained. The average fiber diameter increases with concentration. A combination of elongated beads and fibers, known as the bead-on-string morphology is also observed under many conditions. The fibrous structure is stabilized at high voltages. The fiber diameter in the electrospun polymer typically exhibits a bimodal distribution. The addition of DMF (N,N-dimethylformamide) to the solution increases the deposition rate significantly and leads to extensive splaying, thereby reducing the fiber diameter to about 150 nm. DSC data indicate that electrospinning may lower the degree of crystallinity in the polymer. The wide of range of structural characteristics that may be obtained in the electrospun polymer make it suitable for many biomedical applications including medical textiles, drug delivery, membrane separation, tissue engineering and organ regeneration.

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: Polymer; polystyrene; electrospinning; rheology. Includes bibliographical references (leaves: 67 - 68 ).

The global spread of COIVD-19, as well as the worsening air pollution throughout the world have brought tremendous attention into the development of materials that can efficiently capture particulate matter (PM). Traditional filters made from fabric, glass fibers, or melt blown fibers exhibit a low efficiency at removing sub-micrometer and nanoscale particles. Additionally, they exhibit limited performance in high humidity, high temperature environments. We suggest that the high porosity of filters composed of nanofibers could provide minimal obstruction to air flow, while their high tortuosity and surface area-to-volume ratio presents an excellent platform for particle capture. Electrospinning is a simple and well-studied process to produce randomly accumulated nano- and micro-scale diameter fibers. The main advantages of electrospun nanofibers include their tunable fiber morphology and diameter under specific electrospinning parameters, as well as the ease of post-process modification. Studies have demonstrated its promising applications ranging from tissue engineering, water purification to wearable electronics. Giving the tunable aspect of the process, various polymers were electrospun with different morphology and fiber diameter which all demonstrated high particle removal efficiency. Cellulose was chosen as the base material for our study since it is the most abundant biopolymer and its affinity for further chemical modification. In this study, the removal of nanoscale particles via in-house fabricated cellulose nanofilters is significantly enhanced by chemically functionalizing the fibers’ surface via the deposition of the bio-inspired glue polydopamine (PDA) and the polycation poly(diallyldimethylammonium chloride) (PDADMAC). Nanofilters were electrospun from cellulose acetate solutions before being regenerated to cellulose via an alkaline treatment. Cellulose nanofilters were then functionalized using only PDA or the codeposition of PDA with PDADMAC. Scanning electron microscope (SEM), Fourier transform infrared spectrometer (FTIR), and high-resolution X-ray photoelectron spectroscopy (XPS) were used to characterize the nanofilters. The effects of filter packing density, filter layering, and surface functionalization on their performance, i.e., their filtration efficiency, most penetrating particle size (MPPS), performance in a high humidity environment, and filter pressure drop were investigated. Furthermore, by introducing hydrophilic and hydrophobic nanofibers within a composite filter structure, the performance of the composite filter remained unchanged even in high humidity.

Anterior cruciate ligament (ACL) ruptures and tears are significant orthopedic problems that result in discomfort and limited mobility. Fully functional tissue engineered ligament replacements are promising alternatives to current graft choices for repair of ACL disruptions. The cell-based approach to construct engineered ligament grafts presented herein involves the culture of mesenchymal stem cells (MSC) on biodegradable, fibrous polymeric scaffolds to promote tissue formation. Multipotent MSCs are advantageous because of their in vitro proliferative capacity and ease of harvest; however; the promotion of MSC differentiation into mature fibroblasts and subsequent extracellular matrix (ECM) development is unknown. The proposed studies utilized three complementary methods to promote differentiation of MSCs: scaffold architecture, mechanical stretch and over-expression of the transcription factor, scleraxis. First, elastomeric scaffolds were fabricated by electrospinning a segmented poly(esterurethane urea) with variations in fiber diameter and fiber alignment. Primary mesenchymal stem cells and the mesenchymal stem cell line, C3H10T1/2, were seeded on these scaffolds and assumed spindle-shaped morphologies and oriented with the direction of fiber alignment. Fiber diameter affected cellular responses, including the expression of ECM genes (e.g. collagen type 1 and decorin) which were elevated on smaller mean fiber diameter scaffolds initially. However, scleraxis gene expression was greatest on larger mean fiber diameter scaffolds at the end of two weeks. Second, cyclic stretch was applied to C3H10T1/2 cells on semi-aligned scaffolds using a novel bioreactor. Cell attachment was verified during and after the application of mechanical stress by confocal microscopy. Cyclic stretch induced cells to assume a highly elongated morphology; however ECM gene expression changes were moderate. Third, forced constitutive expression of scleraxis was accomplished by nucleofection of C3H10T1/2 cells. Transient mRNA expression, accumulation of the gene product in the cell nucleus, and cell death were observed. Future work will seek to refine the experimental methods, including the development and testing of an inducible scleraxis transgene and the application of longer periods of mechanical stimulation. Finally, these complementary approaches may be combined to further extend this work in pursuit of directed differentiation of stem cells and the ensuing generation of a robust tissue graft. / Ph. D.

Electrospinning process is a simple and widely used method for producing polymeric nanofibres. However, despite its popularity, significant challenges remain in controlling the fibre deposition due to the complex nature of electrospinning process. The process is renowned for its chaotic motion of fibre deposition, also known as the whipping instability. This instability is caused by electrostatic and fluid dynamics interactions of the charged jet and it is partly responsible for the thinning of the fibres into nanoscale diameters. Due to the instability, an electrospinning process typically deposits random orientated fibres in a circular deposition area. Furthermore, there is no control over the location where the fibres land on the collector electrode except that the fibres always travel through the shortest trajectory between the source and the collector electrodes. In this study, an alternative controlled deposition technique was proposed based on electric field manipulation (EFM). The main hypothesis of this study is that a consistent and repeatable method of controlled deposition can be achieved by using EFM. EFM was achieved by introducing a pair of charged auxiliary electrodes positioned adjacent and perpendicular to the fibre deposition direction. The applied voltage of either direct current (dc) or time-varying (ac) voltage at the auxiliary electrodes act as control to influence the spatial location and size of the deposition area. Samples were produced on black paper substrates and scanned into greyscale images. An image analysis technique was developed to measure the shift and size of the deposition area. A computer simulation was used to calculate the electric field strength and to simulate the behaviour of fibre response based on the trajectory of a charged particle. An image analysis based on greyscale intensity measurement was also developed to examine the uniformity of the deposition area. Finally, fibre characterisation was carried out to examine the fibre morphology, diameter, and orientation based on scanning electron micrographs. The results from this study showed that EFM can provide a consistent and repeatable control of the deposition area. When the auxiliary electrodes were independently charged with two dc voltages, it was observed that the deposition area moved away from the most positive electrode. The magnitude of shift of the deposition area was found to increase linearly with voltage difference between the auxiliary electrodes. Furthermore, the aspect ratio of the deposition area (ratio of width over height) decreased linearly with base voltage i.e. lower of the two auxiliary electrode voltages. These two controls were found to act independently from each other and can be described as two separate controls i.e. voltage difference for spatial location and base voltage for aspect ratio of the deposition area. A similar response was observed in simulation i.e. the particle moved away from the most positive electrode. Simulation results also showed that the x-axis component of the electric field (Ex) was responsible for the shift in location and the reduction of aspect ratio of the deposition area. When the auxiliary electrodes were charged with two antiphase time-varying voltages, continuous scanning of the electrospinning jet was observed producing a wide electrospun fibre mat. It was first thought the smooth oscillation of a sine wave would produce a more uniform deposition pattern compared to a triangle wave, but the results showed otherwise. The inferior uniformity of the sine wave sample was found due to the variability of the jet scanning speed when compared to the constant speed achieved when using a triangle wave. It was also observed that the deposition pattern can be further improved by using two clipped triangle wave voltages. The results open up the possibility for further exploiting the control voltage to achieve the desired deposition pattern. Two case studies were presented to demonstrate the applicability of the technique in real electrospinning applications. In the first case study, it was demonstrated that the continuous scanning of electrospinning jet was capable of eliminating the stripe deposition pattern which is commonly associated to a multi-spinneret electrospinning system. In the second case study, it was found that the alignment and distribution of aligned fibres in a gap electrospinning system can be improved by using the EFM technique. A new technique was also introduced to produce a multi-layer orientated fibre construct. These application examples showed that the EFM technique is ready for the production of engineered electrospun fibre constructs. This would extend the use of electrospun fibres to applications which is currently limited by geometrical constraints of the fibre constructs.

The effects of fundamental solution parameters including polymer molecular weight, concentration and solvent on electrospinning of polystyrene were investigated. Each parameter was found to play a vital role in determining the morphology of beads and fibers. For dilute to semi-dilute solutions, a wide range of bead structures including wrinkled beads, cups, dishes, and toroids were observed when a volatile solvent, tetrahydrofuran, was used. Various rheological regimes where these structures may be obtained were identified. The morphological transition from bead to fiber was characterized by two critical concentrations, Ci and Cf, at which incipient and complete fibers may be observed respectively. These values were determined as a function of molecular weight. A comparison with the models proposed in the literature indicated that solvent evaporation may play an important role in jet stabilization. The fiber diameter and distribution was found to decrease significantly with molecular weight at the critical concentration, Cf. The use of N,N-dimethylformamide, a solvent with relatively high dielectric constant, also resulted in an appreciable reduction in fiber diameter and improved uniformity. The observation of solution jet evolution during the process with high speed camera (2000 frames/s) indicated that solvents have a significant influence on the jet breakdown behavior. Two types of behavior were identified based on the extent of extensional flow, bending instability, and the number of secondary jets. Solvents with high dielectric constant were found to induce extensive bending instability, which resulted in extremely fine microstructures of electrospun polymer.